Chapter 14: Compounds with Oxygen, Sulfur, and Halogens – Alcohols, Phenols, Ethers, Thiols, and Alkyl Halides

Oxygen, Sulfur, and Halogen Containing Compounds

Introduction

This chapter introduces the structure, nomenclature, properties, and reactions of organic compounds containing oxygen, sulfur, and halogens, focusing on alcohols, phenols, ethers, thiols, and alkyl halides. These functional groups are central to GOB Chemistry and have important biological and industrial applications.

Alcohols, Phenols, and Ethers

Definitions and Structures

• Alcohol: An organic compound with a hydroxyl (–OH) group bonded to a saturated, alkane-like carbon chain. Example: ethanol.

• Phenol: Contains a hydroxyl group bonded directly to an aromatic, benzene-like ring. Example: phenol.

• Ether: Contains an oxygen atom bonded to two alkyl or aryl groups (R–O–R). Example: diethyl ether.

Example structures: Ethanol (CH3CH2OH), Phenol (C6H5OH), Diethyl ether (CH3CH2OCH2CH3).

Hydrogen Bonding and Physical Properties

• Hydrogen bonding occurs in alcohols and water due to the highly electronegative oxygen atom bonded to hydrogen.

• Alcohols have higher boiling points than ethers and alkanes of similar molecular weight due to hydrogen bonding.

• Ethers do not form hydrogen bonds with themselves, resulting in lower boiling points compared to alcohols.

Example: Ethanol (bp 78.5°C) > Dimethyl ether (bp –23°C) > Propane (bp –42°C).

Hydrogen Bonding in Alcohols and Water

• Alcohols can form hydrogen bonds with water, increasing their solubility.

• Diols (compounds with two –OH groups) can form multiple hydrogen bonds, leading to even higher boiling points and water solubility.

Naming Alcohols

Common Names

• Alcohols with one –OH group are named by identifying the alkyl group and adding the word alcohol.

• Examples: Methyl alcohol (CH3OH), Propyl alcohol (CH3CH2CH2OH), Butyl alcohol (CH3CH2CH2CH2OH).

IUPAC Naming Rules

• Find the longest carbon chain containing the –OH group.

• Number the chain from the end nearest the –OH group.

• Replace the -e ending of the parent alkane with -ol.

• Indicate the position of the –OH group with a number.

• For cyclic alcohols, add -ol to the parent cycloalkane name.

Examples: 1-propanol (CH3CH2CH2OH), 2-methylcyclohexanol.

Diols and Polyols

• Compounds with two or more –OH groups are named as diols, triols, etc.

• Numbers indicate the positions of each –OH group.

Example: 1,2-ethanediol (ethylene glycol), HOCH2CH2OH.

Classification of Alcohols

• Primary alcohol: –OH group attached to a carbon bonded to one other carbon.

• Secondary alcohol: –OH group attached to a carbon bonded to two other carbons.

• Tertiary alcohol: –OH group attached to a carbon bonded to three other carbons.

Physical Properties of Alcohols

Solubility and Boiling Points

• Alcohols with up to 12 carbons are generally liquids.

• Methanol and ethanol are miscible with water and many organic solvents.

• Diols and polyols have higher boiling points and greater water solubility due to multiple hydrogen bonds.

Example: 1,4-butanediol is more water soluble than 1-butanol.

Reactions of Alcohols

Dehydration Reaction

• Alcohols can lose water when heated with a strong acid, forming alkenes.

• The major product is the alkene with the greater number of alkyl groups attached to the double-bonded carbons (Zaitsev's rule).

Equation: $\text{Alcohol} \xrightarrow{\text{acid, heat}} \text{Alkene} + \text{Water}$

Oxidation of Alcohols

• Primary alcohols oxidize to aldehydes, then to carboxylic acids.

• Secondary alcohols oxidize to ketones.

• Tertiary alcohols do not oxidize under normal conditions.

Equations: $\text{Primary alcohol} \xrightarrow{[O]} \text{Aldehyde} \xrightarrow{[O]} \text{Carboxylic acid}$ $\text{Secondary alcohol} \xrightarrow{[O]} \text{Ketone}$

Phenols

Properties and Uses

• Phenol is a weak acid and was historically used as an antiseptic.

• Alkyl-substituted phenols are less toxic due to lower absorption.

• Phenols have higher melting and boiling points than similar alkylbenzenes due to hydrogen bonding.

• Phenols are less soluble in water than alcohols.

Example: Tyrosine (an amino acid), eugenol (in cloves), urushiol (in poison ivy).

Acidity of Alcohols and Phenols

• Alcohols and phenols are weakly acidic due to the polarized –OH hydrogen.

• Phenols are about 10,000 times more acidic than alcohols (pH ~5–6).

• Phenoxide ion is produced by reaction of phenol with aqueous sodium hydroxide.

Equation: $\text{Phenol} + \text{NaOH} \rightarrow \text{Sodium phenoxide} + \text{H}_2\text{O}$

Ethers

Structure and Naming

• Ethers have two alkyl or aryl groups bonded to the same oxygen atom.

• Named by identifying the alkyl groups and adding the word ether.

• Common examples: Ethyl methyl ether, Diethyl ether.

Physical Properties

• Ethers do not form hydrogen bonds with themselves, resulting in lower boiling points than alcohols but higher than alkanes.

• Ethers are good solvents for reactions where no –OH groups are needed.

• Ethers are highly flammable and can form explosive peroxides when exposed to air.

Thiols and Disulfides

Structure and Naming

• Thiols (mercaptans) contain an –SH group.

• Named by adding thiol to the parent hydrocarbon name.

• Example: 3-methyl-1-butanethiol.

Reactions of Thiols

• Thiols react with mild oxidizing agents to form disulfides (RSSR).

• Disulfide bonds are important in protein structure (e.g., cysteine).

Equation: $2\text{RSH} \xrightarrow{[O]} \text{RSSR} + 2\text{H}_2$

Alkyl Halides

Structure and Naming

• Alkyl halides (R–X) are compounds with a halogen atom bonded to an alkyl group.

• Named by listing the alkyl group followed by the halogen name (e.g., methyl bromide).

• IUPAC names treat the halogen as a substituent on the parent alkane (e.g., 2-bromo-5-methylhexane).

Stereochemistry and Chirality

Chiral Centers and Enantiomers

• Stereochemistry studies the spatial arrangement of atoms in molecules.

• Isomers: Compounds with the same molecular and structural formulas but different atom arrangements.

• Stereoisomers: Isomers that cannot be interconverted by rotation around single bonds.

• Chiral molecules: Molecules with non-superimposable mirror images (enantiomers).

• A chiral center is typically a carbon atom bonded to four different groups.

Example: L-carvone (in spearmint) and D-carvone (in caraway) are enantiomers with different biological activities.

Properties of Enantiomers

• Enantiomers have identical physical properties except for their effect on plane-polarized light and biological activity.

• Many drugs are chiral, and only one enantiomer may be biologically active.

Summary Table: Comparison of Alcohols, Phenols, and Ethers

Compound Type	Functional Group	Hydrogen Bonding	Boiling Point	Water Solubility
Alcohol	–OH on alkyl	Yes (strong)	High	High (small alcohols)
Phenol	–OH on aromatic	Yes	High	Moderate
Ether	R–O–R	No (between molecules)	Low	Moderate

Additional info: Stereochemistry and chirality are important in biochemistry and pharmaceutical chemistry, as the activity of drugs and biomolecules often depends on their three-dimensional arrangement.